The present invention relates generally to semiconductor device manufacturing and more particularly to a system and a method for depositing films.
In the semiconductor industry, there is a continuing trend toward high device densities. To achieve these high device densities, small structures on semiconductor wafers are required. These small structures create high demands for precision in semiconductor processing.
One answer to these demands is to improve in situ process monitoring. In situ process monitoring allows the progression of process steps to be monitored for the purposes of detecting process endpoints and/or controlling process variables such as input power, gas pressure, temperature, and flow, whereby the consistency of process outcomes can be improved. In situ process monitoring, however, can be costly, can be a source of process failure and downtime, and can result in wafer contamination.
An example of a process where there is a need for in situ process monitoring and control is metal film deposition. Metal film deposition is an important step in semiconductor device manufacturing. Deposited metal films are used to form interconnect lines, bus structures, Schottky barriers, ohmic contacts or other structures. Deposition of an appropriate thickness of metal film is often important. For example, if too thin a metal film is deposited, an interconnect line formed from that film may be unacceptably resistive or may have a greater likelihood of becoming an open circuit either during subsequent processing steps or during the normal operation of the device. If too thick a metal film is deposited, deposition time is unduly extended and the film thickness may be in excess of the tolerances of later processing steps. The process endpoint, where the metal film has reached the desired thickness, varies from run to run due to uncontrolled variations of process variables.
A quartz crystal microbalance has been experimentally tested to detect the end point in a metal film deposition process. The microbalance is placed adjacent the semiconductor substrate on which the metal film is being deposited. Based on the variation in the resonant frequency of the quartz crystal with weight of film that deposits on the quartz, the thickness of the deposited film is estimated. When the thickness reaches the target thickness for the wafer, the process is terminated. Unfortunately, the deposition rate on the quartz crystal microbalance can differ from that on the adjacent substrate. In addition, it has been recognized that the reliability of the quartz crystal microbalance measurements varies from run to run. Thus, the quartz crystal microbalance has not been used extensively for deposition process endpoint detection. In general, there remains an unsatisfied need for effective systems and methods for in situ process monitoring and control in semiconductor device manufacturing.
The following presents a simplified summary of the invention in order to provide a basic understanding of some of its aspects. This summary is not an extensive overview of the invention and is intended neither to identify key or critical elements of the invention nor to delineate its scope. The primary purpose of this summary is to present some concepts of the invention in a simplified form as a prelude to the more detailed description that is presented later.
The present invention provides a system and process for depositing films, wherein an acoustic microbalance is used for process monitoring and/or control. The acoustic microbalance is placed in a deposition chamber and may optionally be mounted on a semiconductor substrate, such as a silicon wafer, on which a film is being deposited. Data from the acoustic microbalance is employed to detect a process endpoint, determine an adjustment to processing conditions for a subsequent batch, and/or provide feedback control over current processing conditions.
One aspect of the invention involves the application of a model or database to correct for differences between the extent of deposition on an acoustic microbalance cantilever and the extent of deposition on a substrate being processed. The model can account for systematic differences and also the dependancy of those differences on factors such as temperature and initial thickness of coating on the microbalance cantilever.
Another aspect of the invention takes a probabilistic approach to employing acoustic microbalance data. The acoustic microbalance data is viewed as evidence of the state of a deposition process (principally the extent of deposition), without necessarily interpreting the acoustic microbalance data to determine the amount of film deposited on the microbalance cantilever. Rather, the microbalance data is employed, optionally together with other process data, as evidence in a probabilistic dependancy model that infers the process state and/or predicts a process outcome. Where the probabilistic dependancy model predicts a process outcome, the model can take into consideration prospective changes in process conditions. The process can then be controlled by selecting the changes predicted to result in the best process An outcome. The model can be trained with data gathered using small perturbations around a base set of processing conditions.
Other advantages and novel features of the invention will become apparent from the following detailed description of the invention and the accompanying drawings. The detailed description and drawings provide exemplary embodiments of the invention. These exemplary embodiments are indicative of but a few of the various ways in which the principles of the invention can be employed.